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參數(shù)資料
| 型號(hào)： | IRU3072C |
| 廠商： | International Rectifier |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | 20-PIN SYNCHRONOUS PWM CONTROLLER/ 3 LDO CONTROLLER |
| 中文描述： | 20引腳同步PWM控制器/ 3 LDO控制器 |
| 文件頁(yè)數(shù)： | 16/24頁(yè) |
| 文件大小： | 332K |
| 代理商： | IRU3072C |
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16
Rev. 1.0
3/25/04
IRU3072
www.irf.com
V
STEPLOAD(SPEC)
= 150mV
I
STEPLOAD(MAX)
= 8A
The required ESR is calculated as:
ESR < 150mV/8A = 18.75m
ESR <
V
RIPPLE(SPEC)
/
I
PK_PK
or
ESR <
V
STEPLOAD(SPEC)
/
I
STEPLOAD(MAX)
Output capacitor selection
The voltage rating of the output capacitor is the same as
the output voltage. Typical available capacitors on the
market are electrolytic, tantalum and ceramic. If electro-
lytic or tantalum capacitors are employed, the criteria is
normally based on the value of Effective Series Resis-
tance (ESR) of total output capacitor. In most cases,
the ESR of the output capacitor is calculated based on
the following relationship:
Depending on which one is the requirement.
In this example:
Select three Sanyo POSCAP 6TPB330M with 6.3V
330
μ
F and 40m
ESR will give about 13m
, which will
meet the specification.
Input capacitor Selection
Input capacitor is dertermined by the voltage rating and
input RMS current. For this application, the input RMS
current is given as:
The input RMS current is estimated as:
I
IN(RMS)
= 8A
×
0.1
×
(1-0.1)
2.4A
Select two Sanyo POSCAP -16TPB47M with 16V, 47
μ
F
and 1.4A ripple current. A 1
μ
H, 1A small input inductor
is enough for the input filer.
Power MOSFET Selection
In general, the MOSFET selection criteria depends on
the maximum drain-source voltage, RMS current and
ON resistance (R
DS(ON)
). For both high side and low side
MOSFETs, a drain-source voltage rating higher than
maximum input voltage is necessary. In the demo-board,
20V rating should be satisfied. The gate drive require-
Where:
V
RIPPLE(SPEC)
is the maximum allowed voltage ripple.
I
PK_PK
is the current ripple.
V
STEPLOAD(SPEC)
is the
maximum allowed voltage
droop during the transient or step load.
I
STEPLOAD(MAX)
is the maximum step load current.
ment for each MOSFET is almost the same. If logic-
level or 3V driver MOSFET is used, some caution should
be taken with devices at very low V
GS
to prevent undes-
ired turn-on of the complementary MOSFET, which re-
sults a shoot-through circuit.
If output inductor current ripple is neglected, the RMS
current of high side switch is given by:
D = V
OUT
/V
IN
= 0.1
I
RMS(HI)
= D
×
I
OUT
= 0.1
×
8A = 2.53A
The RMS current of low side switch is given as:
For low side MOSFET, if it is driven by 5V, a logic gate
driver MOSFET is preferred. For R
DS(ON)
of the MOSFET,
it should be as small as possible in order to get highest
efficiency. A logic driver MOSFET such as IRF7460 from
International Rectifier in a SOIC 8-pin package,
R
DS(ON)
=10m
, 20V drain source voltage rating and 12A
I
DS
is selected for high side and low side MOSFET.
Power Dissipation for MOSFETs
The power dissipation for MOSFETS typically includes
conduction loss and switching losses. For high side
switch, the conduction loss is estimated as:
P
COND(HI)
= D
×
I
OUT
×
I
OUT
×
R
DS(ON)MAX
The R
DS(ON)
has to consider the worst case. In the
datasheet of IRF7460:
The switching loss is more difficult to calculate because
of the parasitic parameters. In general, the switching
loss can be estimated by the following:
tr is the rising time and tf is the falling time. From IRU3072
datasheet: tr=50ns and tf=50ns
The total disspation for the high side switch is:
P
D(HI)
= P
SW(HI)
+P
COND(HI)
2W
For low side switch, most of the loss are conduction
loss. The low side switch power dissipation is:
P
SW
= 0.5
×
V
DS
×
I
OUT
×
(tr+tf)
×
F
S
P
SW(HI)
= 0.5
×
12V
×
8A
×
(50ns+50ns)
×
400KHz
P
SW(HI)
1.92W
P
D(LO)
P
COND(LO)
= (1-D)
×
I
OUT
×
I
OUT
×
R
DS(ON)MAX
P
D(LO)
P
COND(LO)
= (1-0.1)
×
8A
×
8A
×
14m
P
D(LO)
P
COND(LO)
= 0.81W
R
DS(ON)MAX
= 14m
@ Vgs = 4.5V
P
COND(HI)
= 0.1
×
8A
×
8A
×
14m
0.09W
I
IN(RMS)
= I
OUT
×
D
×
(1-D)
D = V
OUT
/V
IN
= 1.2V/12V
0.1
I
RMS(HI)
= 1-D
×
I
OUT
= 1-0.1
×
8A = 7.6A
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